The present invention relates generally to x-ray systems, and more particularly to a method and apparatus for supplying and directing electrons on a target within an imaging tube.
There is a continuous effort to increase x-ray imaging system scanning capabilities. Customers desire the ability to perform longer scans at high power levels. The increase in scan time at high power levels allows physicians to gather images and constructions in a matter of seconds rather than several minutes as with previous x-ray imaging systems. Although the increase in imaging speed provides improved imaging capability, it causes new constraints and requirements for the functionality of the x-ray imaging systems.
X-ray imaging systems include an imaging tube. The imaging tube generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large direct current (DC) voltage potential is created across the vacuum gap allowing electrons to be emitted from the cathode to a target within the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons, in the form of an electron beam, are accelerated by the high voltage potential and impinge on the target, whereby they are abruptly slowed down to emit x-rays. The deceleration of the high energy electrons in the target solid produces a large amount of heat.
High-voltage, high power imaging tubes have several disadvantages. High-voltage, high power imaging tubes contain a complex vacuum enclosure that is carefully manufactured to properly prepare internal surfaces and volumes of material enclosed within the imaging tube. Many of the most critical surfaces include a cathode cup and an anode target, which are subject to very high electric field stress. A costly process referred to as “seasoning” prepares the more critical internal surfaces. Seasoning includes removing air from within an imaging tube and heating critical surfaces as well as the imaging tube enclosure, to exhaust any existing gases in the internal surfaces. Seasoning specifications vary between applications due to geometry and material composition differences.
Surfaces exposed to large electromagnetic field gradients must be specially treated due to added stress. Highly stressed surfaces are located within the vacuum enclosure at high discharge locations such as from cathode to anode, anode to frame, and cathode to frame. Any evolution of gas or surface asperity or blemish on any of these surfaces is a precursor to high-voltage activity. High-voltage activity sometimes referred to as “spit” activity, is further described below.
Another disadvantage of imaging tubes is that electric field gradients along with high vapor pressures within the imaging tube can cause high-voltage instability. The electric field gradients are present at an anode target when an electron beam is incident. The high vapor pressures are due to the following gas species: background gas, surface-absorbed gas, target bulk absorbed gas, or track material atoms. Background gas is residual gas remaining in the imaging tube after the exhaust process. Surface-absorbed gas and target bulk absorbed gas refer to gases remaining within surfaces of the imaging tube internal componentry. Track material atoms, refers to atoms on the surfaces that are evaporated into the gases of the imaging tube. The high vapor pressures are at pressures of approximately 10−4 mbar, which is undesirable compared to preferred operating gas pressure of 10−7 mbar. The gas species provide ionization targets for incident electron flux. Charged ions and excess electrons produce a low impedance path between high DC potentials of the anode and the cathode. The DC potentials and the electromagnetic field gradients cause spit activity within the imaging tube. Spit activity refers to ignited ions generated by the high-pressured gas that arc to internal surface asperities. Spit activity temporarily causes the x-ray imaging system to malfunction or shutdown, which is especially undesirable during a medical diagnosis.
Also due to the aforementioned and other traditional imaging tube characteristics, the materials and gases used to manufacture the imaging tubes can be limited and extensive. For example, due to backscattering of electrons in traditional imaging tubes, a copper electron collector is used between the cathode and the anode to remove heat. Another limiting example is the inability to use low-Z gases due to increasing vapor pressure during imaging tube use, caused by spit activity and arcing. Low-Z gases can enhance heat transfer between an anode and an imaging tube frame. The limitation of available gases and existing vacuum environment also requires vacuum compatible lubricants for use on anode bearings. The limitation on lubricants limits the ability to produce a more quiet, reliable, and inexpensive anode bearing.
Therefore, it would be desirable to provide an improved method and apparatus for supplying and directing electrons on a target within an imaging tube that eliminates the need for seasoning, provides increased high-voltage stability, and increases imaging system engineering flexibility in choices of materials and gases within an imaging tube.